Handbook of Enology, Volume 2. Pascal Ribéreau-Gayon

16.6

TABLE 1.6 Demonstration of Interactions Between Organic Acids and the Effect of Alcohol on the Buffer Capacity of Binary Combinations (Dartiguenave et al., 2000b)

Medium	Buffer capacity (mEq/l)	Composition of equimolar mixes of two acids Total acid concentration (40 mM)
Tartaric acid Malic acid	Tartaric acid Succinic acid	Tartaric acid Citric acid
Water	Experimental value	21	20	23.5
	Calculated value	25.7	25.7	26.3
	Difference (Calc. − Exp.)	4.7	5.7	2.8
EtOH (11% vol.)	Experimental value	18.3	20.1	29
	Calculated value	24	23.3	24
	Difference (Calc. − Exp.)	5.7	3.2	−5
Effect of ethanol	(EtOH − H2O) Exp.	−2.7	0.1	5.5

      These findings indicate that the acids interact among themselves and with alcohol, compensating for the decrease in buffer capacity of each individual acid when must (an aqueous solution) is converted into wine (a dilute alcohol solution). From a purely practical standpoint, the use of citric acid to acidify dosage liqueur for bottle‐fermented sparkling wines has the doubly positive effect of enhancing the wine's aging potential while maintaining its freshness on the palate.

Table 1.9 shows the changes in buffer capacity in successive pressings of a single batch of Chardonnay grapes from the 1995 to 1996 vintages, at the main stages in the winemaking process.

The demonstration of the effect of alcohol and interactions among organic acids (Tables 1.6–1.8) led researchers to investigate the precise contribution of each of the three main acids to a wine's buffer capacity in order to determine whether other compounds were involved. The method consisted in completely deacidifying a wine, containing all of its malic acid, by crystallizing and precipitating the double calcium tartromalate salt. After this deacidification, the Champagne base wine had a residual total acidity of only approximately 0.5 g/l as H₂SO₄, whereas the sample's buffer capacity was still 30% of the original value. This shows that organic acids are not the only compounds involved in buffer capacity, although they represent 90% of total acidity.

TABLE 1.7 Demonstration of Interactions Between Organic Acids and the Effect of Alcohol on the Buffer Capacity of Ternary Combinations (Dartiguenave et al., 2000b)

		Composition of equimolar mixes of three acids (13.3 mM) Total acid concentration (40 mM)
Medium	Buffer capacity (mEq/l)	Tartaric acid Malic acid Succinic acid	Tartaric acid Malic acid Citric acid
Water	Experimental value	9.4	11.6
	Calculated value	25.4	25.5
	Difference (Calc. − Exp.)	16.0	13.9
EtOH (11% vol.)	Experimental value	21.7	26.4
	Calculated value	22.8	23.2
	Difference (Calc. − Exp.)	1.1	−3.2
Effect of ethanol	(EtOH − H2O) Exp.	12.3	14.8

TABLE 1.8 Effect of Hydroxyl Groups in the Structure of the Four Carbon Diacids on Buffer Capacity (mEq/l) (Dartiguenave et al., 2000b)

Medium	1 Hydroxyl group	2 Hydroxyl groups
Malic acid	Succinic acid	Δ (Mal. − Suc.)	Tartaric acid	Malic acid	Δ (Tart. − Mal.)
Water	23.8	23.4	0.4	29	23.8	5.2
11% vol. dilute alcohol solution	22.0	20.6	1.4	25.9	22	3.9

TABLE 1.9 Changes in the Buffer Capacity of Must from Different Pressings of Chardonnay Grapes at Various Stages in the Winemaking Process (Buffer Capacity Is Expressed in mEq/l) (Dartiguenave, 1998)

	Cuvée	Second pressing
	1995	1996	1995	1996
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